In general, in a workpiece such as a cylinder head that constitutes an engine, a plurality of valve holes to which intake valves and exhaust valves are attached are provided for a single cylinder (see, for example, Patent Document 1). When machining the plurality of valve holes in the cylinder head, it is preferable that the valve holes are machined by a small number of processes and with high precision.
In order to simultaneously machine a plurality of valve holes, it is considered to use a so-called gang head (multi-spindle head), in which, for example, a plurality of tools for performing reamer machining is rotatably provided. However, it is difficult to machine a plurality of intake valve holes and a plurality of exhaust valve holes which are formed on one cylinder with single processing, since the valve holes are proximately adjacent to each other. In order to solve this, a plurality of multi-spindle heads for intake valve holes and a plurality of multi-spindle heads for exhaust valve holes are provided, and valve holes that are to be machined are machined using one of the multi-spindle heads that is suitable to the position of the valve holes that are to be machined. Therefore, there are problems in that the number of the multi-spindle heads increases, the operation becomes complicated, and the steps of the machining process increase.
In addition, a clamp device that clamps the cylinder head with respect to the machine tool is used when machining the valve holes in the cylinder head as described above.
It is preferable that a clamp device shifts (moves) a workpiece with respect to the machine tool in the state in which the workpiece is clamped. However, when an actuator for changing the amount of shift is provided, there is a problem in that the configuration of the apparatus becomes complicated.